Startle responses elicited by whiplash perturbations

This article was just published online in the Journal of Physiology. Free full text is available.

Blouin JS, Inglis JT, Siegmund GP. Startle responses elicited by whiplash perturbations. J Physiol 2006;[epub ahead of print; published online June 7 2006]

Abstract: The human startle response produces muscle contractions throughout the body but the most brisk and synchronised contractions appear in the neck muscles. This response, which is greatest with the first exposure to a startling stimulus, could produce excessive and inappropriately directed muscle contractions that could explain the higher incidence of whiplash injuries in people who are unprepared for the collision. This study seeks neurophysiological evidence of startle responses in the neck muscles of 120 healthy subjects exposed to between 1 and 16 rear-end impacts or forward perturbations of different speeds. Startle responses were quantified by the synchronous electromyographic (EMG) activity between 10 and 20 Hz in bilaterally-homologous sternocleidomastoid, scalene and cervical paraspinal neck muscles. Coherence analyses of EMG from the left and right muscles were used to estimate synchrony for:
i) the first unexpected trial,
ii) subsequent habituated trials and
iii) the superposition of habituated trials and a loud acoustic stimulus (40 ms, 124dB sound). The peak in coherent EMG activity between contralateral muscle pairs in the 10 to 20 Hz bandwidth was related to startle. Synchrony in this bandwidth was observed between the left and right muscles during the first impact or whiplash-like perturbation. This synchrony decreased significantly in the habituated trials, but reappeared when the loud acoustic stimulus was introduced. Its presence in the first trial indicates that startle is part of the neuromuscular response to an unexpected rear-end impact. This startle component of the neuromuscular response could play a role in the aetiology of whiplash injuries.
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